Environmental Biology & Genetics Genotype & Phenotype M r G R D a v i d s o n

Environmental Biology & Genetics

Genotype & Phenotype

Mr G R

Mr G R

Davidson

Davidson

Friday, April 21, 2023Friday, April 21, 2023 22Mr G DavidsonMr G Davidson

Alleles

• Genes control the characteristics of an organism, e.g. flower colour in peas.

• Alleles are different forms of the same gene.– An allele is a different form of that gene,

e.g. red flower or white flower.


Examples of alleles

OrganisOrganismm

GeneGene Different AllelesDifferent Alleles

Pea plant Height Tall or dwarf

Humans Blood type A or B or O

Fruit fly Wing type Normal or vestigial

MaizeSeed colour

Purple or yellow


Alleles

• Most genes normally have at least 2 forms. (Some have more than 2)

• Each body cell contains 2 alleles for every characteristic.

• This is because the cell contains 2 of each type of chromosome and the alleles are found at the same place on each of the two chromosomes.


Alleles

Pair of identical chromosomes

Allele of a particular gene

Second allele of this particular gene, which may be the

same or it may be different


Alleles

• When gametes (sex cells) are being formed, the pair of chromosomes will be split up, each gamete only getting one chromosome.

• E.g. This chromosome would go to one gamete

This chromosome would go to the

other gamete


Alleles

• This means that when one gamete fertilises another, the resulting zygote will have 2 alleles, one from each gamete.

• The appearance of the organism depends on which of the alleles is dominant or recessive.


Alleles

• A dominant allele will always show up in the appearance of an organism.

• A recessive allele will only show up if it is paired with another recessive allele.

• When describing an allele, for convenience, we usually give it a symbol, generally the first letter of the dominant allele.


Alleles

• If it is dominant it gets the capital letter and if it is recessive it gets the lower case of the same letter.

• E.g. in pea plants tall is dominant over dwarf, and so the tall allele is given the letter T, and the dwarf allele the letter t.

• The genotype of the organism is the alleles it carries for that particular characteristic.


Alleles

• TT is a genotype and the plant will be tall.• Tt is a genotype and the plant will be tall,

because the T is dominant over the t.• tt is a genotype and the plant will be

dwarf.• What the plant actually looks like (how

the genes are expressed) is called its phenotype.


Genotypes & Phenotypes

• If both alleles are the same in the genotype of an organism, it is said to be true-breeding or homozygous, i.e. TT or tt.

• If the alleles are different in the genotype of the organism, it is said to be heterozygous, i.e. Tt.


Genotypes & Phenotypes

Genotype

Phenotype

Description

TT Tall Homozygous

Tt TallHeterozygous

tt Dwarf Homozygous


Monohybrid Inheritance

• In pea plants, red flower colour (R) is dominant to white flower colour (r).

• If we were to cross a homozygous red plant with a white plant, we would set the cross out as follows:


• To find out which characteristic is dominant we carry out a test cross.

• We cross true breeding (homozygous) strains of the two alleles.

• We refer to the first generation as the P (parent) generation and the resulting generations as F1 and F2 (first and second filial generations)

Friday, April 21, 2023Friday, April 21, 2023 Mr G DavidsonMr G Davidson 1414



Red x WhiteParent PhenotypeParent Genotype

RR x rr

Gametes

R R r r

F1 Genotype

Rr Rr Rr Rr

F1 Phenotypes

Red Red Red Red

All the offspring are red & heterozygous.


• To complete the test cross the F1

generation are allowed to breed together producing a ratio of 3 dominant : 1 recessive trait

• This is split up into:• 1 dominant homozygous:2 dominant

heterozygous :1 recessive homozygous.




Red x RedF1 Phenotype

F1 Genotype Rr x Rr

Gametes

R r R r

F2 Genotype

RR

Rr rR rr

F2 Phenotypes

Red Red Red White

¾ of the offspring are red & ¼ are white.



• This type of inheritance was first studied in the 19th Century by an Austrian monk called Gregor Mendel.

• He only studied one clear characteristic at a time in breeding experiments which we call crosses.

• The crosses are worked out using a Punnett square.

• E.g. in mice black coat colour is dominant over albino.


Monohybrid CrossesWe always set out a cross the same way:Parents (P) Black x Albino

BB bb

Gametes

B bGametes

The gametes for one parent go along the top.

B B

The gametes for the other parent go down the side.

b

b

We can then complete the square.

Bb Bb

Bb Bb

The results of this cross are that all the F1 are black.The results of this cross are that all the F1 are black.

B b


Monohybrid CrossesIf we now cross 2 of the F1 generation:

F1 Black x BlackBb Bb

Gametes

B BGametes


B b


B

b


BB Bb

Bb bb

The results of this cross are that 3 of the F2 are black and 1 is The results of this cross are that 3 of the F2 are black and 1 is albino.albino.

b b



• We can then identify the genotype of individuals who have black fur by back-crossing them with albino mice.

• This is called a test backcross and would look like the following.


Monohybrid CrossesParents (P) Black x Albino

Bb bb

Gametes

B b

Gametes


B b


b

b


Bb bb

Bb bb

The results of this cross are that half of the The results of this cross are that half of the F1 are black and half are albino.F1 are black and half are albino.

b b


Monohybrid Crosses

• Your answer should always have the parental genotypes.

• Your answer should always have the parental gametes.

• Your answer should always have a completed punnett square.

• Your answer should always have the F1 phenotypes and the ratio they occur in.


Family Trees

• Family tree diagrams can be used to show the transmission of alleles over a number of generations.

• The following family tree shows how the “ability to roll the tongue” allele is transmitted through 3 generations.


Family Trees= male

= female

= ability to roll tongue

= inability to roll tongueGranny GrannyGrandad Grandad

Dad

Aunt Mum

Uncle

Brother BrotherSister SisterSince each organism receives 2 pieces of information for this

characteristic, we now need to work out the GENOTYPE.


Family TreesRR rr rrrr

rr

rrrr

rr

Rr Rr

RrRr


Co-Dominance

• It is possible for 2 alleles to have the same level of dominance, and in this case they are said to be co-dominant.

• In this case the offspring usually have a phenotype consisting the characteristics of both parents.

• E.g. If red and a white short horn cattle are crossed, the offspring has red and white hairs, resulting in a roan cow (looks pinkish).


Co-Dominance

Parents (P) Redx WhiteRR WW

Gametes

R WGametes

R R

W

W

RW RW

RW RW

In this case all In this case all of the of the

offspring are offspring are “roan”.“roan”.

R W


Variation

• There are 2 types of variation shown in organisms:– Discontinuous variation– Continuous variation


Variation

• Discontinuous variation shows distinct clear-cut differences, and is controlled by only one gene.

• Discontinuous variation is usually displayed in a bar chart.


Discontinuous Variation

e.g. eye colour


Discontinuous Variation

e.g. blood type


Discontinuous Variatione.g. ear lobes

Attached lobe Unattached lobe


Variation

• Continuous variation shows a range of differences, and is controlled by more than one gene.

• These differences can usually be measured.

• Continuous variation is usually displayed in a histogram.


Continuous Variation

e.g. Shell diameter in limpets.



e.g. human hand span



e.g. Height


Environmental Impact

• The final phenotype of an organism is influenced by variation in the environment in which the organism lives.

• Identical twins have the same genotype but may differ because of:– Diet (eating different food)– Activities (if one exercises a lot)– Climate (if one moves to another climate)GENOTYP

E+ENVIRONMENT PHENOTYPE


Natural Selection

• Most organisms produce far more offspring than are able to survive.

• This leads to a struggle for survival and many offspring will die before reaching an age where they can reproduce.


Natural Selection

• There are several reasons why offspring do not survive.– Starvation– Eaten by predators– Disease– Exposure


Natural Selection

• The ones which survive often have a better phenotype, suited to the environment, and they can pass on their genes.

• This is called “survival of the fittest”.


Natural Selection

• Examples of genes which can be advantageous include:– Speed– Aggression– Resistance to disease– Coat thickness– Coat colour (better camouflage)– Quicker reactions


Natural Selection

• Only the organisms better adapted to survive in their environment go on to reproduce.

• This can lead to the appearance of a new species.

• This is called evolution.


Peppered Moth

• The peppered moth is an excellent example of natural selection.

• It has a lightly speckled body colour which provides camouflage against lichens which grow on tree barks.

• This means the birds can’t see them.


Peppered Moth

• There is also a black variety of the peppered moth. (MELANIC)

• The black moths are easily seen and eaten by the birds.

• However, during the industrial revolution, the burning of coal produced a lot of black sooty smoke.


Peppered Moth

• This killed the lichens on the trees and turned them black.

• This allowed the black moths to be easily camouflaged, and so they survived to reproduce more black moths, and so the population of black moths increased rapidly.

• However, the speckled moth became easier for the birds to see, and so their numbers were quickly reduced.


Peppered Moth

• This only happened in industrial areas where the pollution was at its highest.

• Today both types of moth survive in different areas and they can still interbreed.– In large industrial areas the black

(melanic) moth is more common.– In rural areas the light speckled moth is

more common.


Peppered Moth

• Evolution has not yet caused these moths to become different species.

• Today, the Clean Air Act is reducing pollution, and this will reduce the numbers of the melanic moth.

• Natural selection allows the moths to survive where they are.

• Natural selection leads to Biodiversity.

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Environmental Biology & Genetics Genotype & Phenotype M r G R D a v i d s o n